Cardiovascular evaluation of a subject may be performed directly (such as by intra-arterial measurements) or indirectly (e.g., by the classic auscultatory method). Because of the dangers associated with the direct method of measurement, the auscultatory method has become nearly universally accepted as the method of choice by medical clinicians and researchers alike.
As is well known, the auscultatory method of determining the blood pressure of a patient makes use of the acoustical disturbances ("Korotkoff sounds" or "K-sounds") produced when blood flows through a compressed (occluded) artery. The physician typically positions an inflatable occluding cuff about the upper arm of the patient and positions the sensing orifice of a stethoscope between the cuff and the arm in the vicinity of the brachial artery. A squeeze-type air pump is used to inflate the occluding cuff to a pressure somewhat greater than the systolic blood pressure of the patient to completely occlude the brachial artery. No sounds are detected by the stethoscope when the artery is fully compressed by the cuff because of the absence of arterial blood flowing through the artery below the point of occlusion.
An air valve is operated by the physician to slowly deflate the cuff and thereby decrease the pressure it exerts upon the walls of the brachial artery while the cuff pressure is continuously monitored (e.g., by a mercury manometer or the like). When the pressure exerted by the cuff falls to slightly below the patient's systolic blood pressure, each cardiac systole produces a head front of blood forcing the walls of the brachial artery to exert a greater force against the occluding cuff than the cuff exerts in return, thereby causing the artery to become temporarily unoccluded. The temporary opening of the brachial artery under the force produced by the cardiac systole produces a Korotkoff sound which can be heard with the stethoscope. The physician notes the cuff pressure when he or she hears the first Korotkoff sound to obtain the systolic blood pressure of the patient.
The artery becomes occluded once again after the head front of blood produced by the cardiac systole passes through the point of occlusion, and remains occluded until the next cardiac systole occurs. A Korotkoff sound is produced for each contraction of the heart (delayed slightly from the time of systole by the time it takes the head front of blood to travel from the heart to the point of arterial occlusion) so long as the cuff pressure is greater than the diastolic blood pressure of the patient. The physician may count the number of Korotkoff sounds occurring during a given time interval to measure the heart rate of the patient.
The physician continues to slowly deflate the cuff until Korotkoff sounds are no longer heard in the stethoscope (indicating that the cuff no longer exerts sufficient pressure on the arterial wall to occlude the artery even during cardiac diastole). The physician notes the cuff pressure at this time as the diastolic blood pressure of the patient.
Non-invasive measurement of blood pressure and heart rate is useful for clinical, experimental and medical research purposes. For instance, such measuring techniques are useful for monitoring surgical patients in the operating or recovery room and for other clinical patient evaluation (such as clinical stress or exercise testing). Classic auscultatory measuring techniques performed by a trained individual can typically be used in clinical settings, but are impractical for other applications, such as the cardiovascular evaluation of mobile patients during their exposure to daily environmental stress or evaluation over extended periods of time. Portable devices capable of automatically obtaining cardiovascular data from an ambulatory subject have therefore been developed to make possible a wide range of cardiovascular research involving, for example, field studies, biofeedback studies, and patient behavioral modification.
A portable blood pressure monitoring system capable of obtaining cardiovascular information about a subject over long periods of time is described in U.S. Pat. No. 4,216,779 to Squires et al (issued Aug. 12, 1980) and a 1979 product information bulletin entitled "Pressurometer III Ambulatory Blood Pressure Monitor Model 1978" published by Del Mar Avionics of Irvine, Calif. The disclosed system non-invasively measures blood pressure and heart rate under electronic control using the auscultation method. An occlusion cuff is automatically inflated at predetermined programmable intervals to a level greater than and dependent upon the last-measured systolic pressure of the subject. The cuff is subsequently deflated at fixed decrements in response to R-waves detected by ECG electrodes, while a microphone senses Korotkoff sounds. The presence or absence of a Korotkoff sound within a preset interval following each heartbeat is used to determine when the cuff pressure (measured by an electronic pressure sensor) equals the systolic and diastolic blood pressures of the subject. Systolic and diastolic blood pressures corresponding to the first and last Korotkoff sounds detected during each cuff inflation-deflation cycle are converted into digital signals and displayed on an electronic display. The systolic and diastolic pressures are also recorded in digital form on a continuously-running portable tape recorder together with signals from the ECG electrodes. After a number of cycles of operation, the magnetic tape is removed from the recorder and inserted into an analyzer for high-speed play-back to obtain a plot of heart rate and accompanying blood pressure readings on a common chart.
U.S. Pat. No. 4,252,127 to Gemelke (issued Feb. 24, 1981) discloses a portable blood pressure device also having an automatically-inflatable occluding cuff, a Korotkoff sound sensor, a cuff pressure sensor, and a tape recorder. The occluding cuff is periodically automatically inflated and subsequently slowly deflated. A tape recorder is turned on when the cuff pressure begins to fall. The output of the Korotkoff sound microphone is gated by a signal produced by pulsations in cuff pressure caused by heartbeats, filtered, buffered, and applied to one input of a summing amplifier. The other input of the summing amplifier is connected to the voltage-encoded output of the cuff pressure sensor. The resulting composite output is modulated by a voltage-to-frequency converter to produce a waveform with constant off-time occurring at a variable frequency. This waveform is recorded on the tape recorder. Respective pressure and K-sound signals are later recovered individually by passing the playback signal from the tape recorder through a comparator, rectifier, frequency-to-voltage converter and filtering network. High-pass filtering eliminates the pressure signal from the demodulated composite signal, leaving the K-sound signals. The pressure signal is likewise recovered by filtering the demodulated composite signal with a low-pass filter.
Graham, M., ".mu.Ps Monitor EKG and Blood Pressure", 24 Electronic Design No. 19, p. 28 (Sept. 13, 1976) discloses a portable electrocardiogram and blood pressure signal monitor which processes information only when deviations are detected. Graham apparently suggested and tried a technique of matching Korotkoff sound frequency characteristics of each K-sound occurring in real time with those of previously-stored Korotkoff sound models corresponding to systolic and diastolic pressure. The 1976 Graham article includes a block diagram of a microprocessor-controlled medical monitor and an extremely brief description of his Korotkoff sound detection technique. The tape recorder as shown in the block diagram is connected directly to the MPU, indicating that data was stored on tape in digital format only.
U.S. Pat. No. 4,270,547 to Steffen et al (1981) discloses a vital signs monitoring system which converts analog measurements of cardiovascular parameters into digital form for display.
Other references disclosing blood pressure measuring devices using a magnetic tape recorder to store cardiovascular measurements include U.S. Pat. No. 4,211,238 to Shu et al (issued July 8, 1980) and U.S. Pat. No. 4,378,807 to Peterson et al (issued Apr. 5, 1983). U.S. Pat. No. 4,228,506 to Ripley et al (issued Oct. 14, 1980) discloses an intelligent data logging printer-plotter for use with a source of time-correlated digital systolic and diastolic blood pressure data and heart rate data.
Another automated non-invasive blood pressure recording device is disclosed in "Validation of the Vita-Stat Automated Blood Pressure Recording Device" by Dembroski et al, reprinted in Herd et al, Cardiovascular Instrumentation: Applicability of New Technology to Biobehavioral Research (National Institute of Health, Bethesda, Md. 1983). The system described in this publication includes an ambulatory data logging module with a removable random access memory pack and a base unit for interfacing the memory pack with a microcomputer. The logging module is programmed and initialized for measurements by loading information from the microcomputer into the memory pack. Cardiovascular measurements are automatically taken at fixed predetermined intervals and recorded in digital form in the memory. After measurements are completed, the memory pack is removed from the logging module and data stored in the memory pack is down-loaded via a data bus to a microcomputer for storing on a floppy disk, printing, analyzing, etc.
The following references disclose other cardiovascular data acquisition devices of possible relevance to the present application:
U.S. Pat. No. 3,654,915 to Sanctuary (1972); PA0 U.S. Pat. No. 4,003,370 to Emil et al (1977); PA0 U.S. Pat. No. 4,397,317 to Villa-Real (1983); PA0 U.S. Pat. No. 4,383,534 to Peters (1983); PA0 U.S. Pat. No. 4,248,241 to Tacchi (1981); PA0 U.S. Pat. No. 4,519,398 to Lisiecki, et al.(1985); PA0 U.S. Pat. No. 4,510,942 to Miyamae, et al (1985); PA0 U.S. Pat. No. 4,475,557 to Hatschek, et al (1984); PA0 U.S. Pat. No. 4,459,991 to Hatschek (1984); PA0 U.S. Pat. No. 4,320,767 to Villa-Real (1982); PA0 U.S. Pat. No. 4,248,242 to Tamm (1981); PA0 U.S. Pat. No. 4,112,929 to Affeldt, et al (1978); PA0 U.S. Pat. No. 3,996,928 to Marx (1976); PA0 U.S. Pat. No. 3,776,228 to Semler (1973); PA0 U.S. Pat. No. 3,550,582 to Wilhelmson (1970); and
Two "BPI 520" advertisements published by Medtek of Carrollton, Tex. in June, 1983;
A paper published by Smith, Hutcheson, Lutz and Hsiao of The University of North Carolina at Chapel Hill entitled "A Microprocessor System To Measure Blood Pressure", Advanced Medical Systems: An Assessment of the Contributions, (10th Annual Meeting of the Society for Advanced Medical Systems, Symposia Specialists, 1979, distributed by Year Book Medical Publishers, Chicago, Ill.) describes a microprocessor-based system which automatically controls occluding cuff pressure and detects Korotkoff sounds only during a short window referenced to the EKG. A microcomputer is used to store and analyze data. See also Obrist & Hutcheson, "The Non-Invasive Measurement of Blood Pressure in Bio-Behavioral Research," presented at the "Working Conference on Cardiovascular Instrumentation: Applicability of New Technology to Bio-behavioral Research," Houston, Tex. (May 16-19, 1982) (addressing methods for accurately identifying Korotkoff sounds); Obrist & Hutcheson, "An Automated Non-Invasive Blood Pressure Monitor: Principle of Operation and Reliability" (1979); and Jennings et al, "Publication Guidelines for Heart Rate Studies in Man," 18 Psychophysiology 226-31, (The Society for Psychophysiology Research, May, 1981).
The following references (believed to be less relevant than those cited above) disclose physiological parameter monitoring systems and/or components for use in such systems:
______________________________________ U.S. Pat. No. Inventor Date ______________________________________ 4,106,495 Kennedy 1978 4,339,800 Woods 1982 4,417,254 Woods 1983 4,422,081 Woods 1983 4,123,785 Cherry, et al. 1978 Re. 29,921 Cherry, et al. 1979 4,073,011 Cherry, et al. 1978 4,513,753 Tabata, et al. 1985 4,483,346 Slavin 1984 4,429,699 Hatschek 1984 4,300,573 Rebbe, et al. 1981 4,120,294 Wolfe 1978 4,108,166 Schmid 1978 4,097,113 McKelvy 1978 3,906,939 Aronson 1975 3,906,937 Aronson 1975 3,896,791 Ono 1975 3,754,545 Weinstein 1973 3,704,708 Iberall 1972 3,557,779 Weinstein 1971 3,542,011 Langenbeck 1970 ______________________________________
Many of the portable blood pressure monitoring systems disclosed in the cited references measure the systolic and diastolic blood pressure and heart rate of a subject automatically over an extended period of time. However, further improvements in such devices are necessary in order to increase reliability and accuracy. For example, presently-available ambulatory blood pressure monitoring systems use analysis algorithms which make decisions in real time based upon each Korotkoff sound as it occurs. The present applicants have discovered that this approach can (and frequently does) lead to erroneous pressure determinations because the measuring system has no way of determining if the signals appearing on the Korotkoff sound channel represent actual Korotkoff sounds associated with the Korotkoff sound envelope occurring between systolic and diastolic cuff pressure points, or whether the signals instead are caused by spurious noise and/or artifacts having frequency characteristics similar to Korotkoff sounds. Real-time signal processing techniques such as noise reduction and dynamic thresholding can partially but not completely eliminate errors introduced by these effects. Moreover, if a patient's Korotkoff sound profile suddenly changes drastically from one measurement cycle to the next, or worse yet, within a period during which the occluding cuff is being deflated, a system using real-time analysis of Korotkoff sounds cannot compensate for the changes in the Korotkoff sound envelope profile, and may determine blood pressure erroneously. Patents may be diagnosed incorrectly due to such errors, since there is no way to tell whether atypical measurements are caused by intermittent cardiovascular problems of the patient or by measurement errors.